Basic principles of 14C AMS

The AMS technique is an extension of conventional isotope ratio mass
spectrometry in which a magnetic field is used to separate the ionised carbon
isotopes by their different masses which are quantified separately. The low natural abundance of 14C (one 14C-atom
in 1012 to 101512C-atoms) prevents the use of
conventional mass
spectrometer measurements, because of much higher abundances of other ions of
the same mass (14N and
molecular ions like 12CH2, 13CH,
...).

In AMS, 14N is eliminated by the initial choice of a negative ion
beam (no stable N-) and, after selection of mass 12, 13, 14 and acceleration to
2.5 MeV, molecular ions are destroyed in a gas
stripper (Ar)
set to strip the negative of a few electrons to an electric charge of
+3. Repeated magnetic and electrostatic filtering reduces interfering
ions to a
level where 14C concentrations in the range 10-12 to 10-15
can be measured.

Schematic of the AMS system: the symbols indicate L a lens, Y-S one pair, XY-S
two pairs of steerers (horizontal and vertical),
BPM a beam profile monitor, FC faraday cups, SL vertical slits, and A aperture (drawing by M.-J. Nadeau).